1. Technical Field
The present invention generally relates to the field of current detection and more particularly, is directed to a circuit for measuring direct current or slowly changing currents utilizing a current transformer.
2. Related Art
As known in the prior art, direct current, or slowly changing currents, can be measured by measuring the corresponding voltage drop across a resistive shunt. A problem with resistive shunts, however, is that they dissipate power. For example, a 100 millivolts drop across a 50 amp shunt will dissipate 5 Watts. Furthermore, in an electrically noisy environment, the usually small voltages which are produced by a resistive shunt are too small to be reliably measured. Commonly available shunts typically generate 50-100 millivolts which yield unreliable results in the noisy environment of electronic power converter. Shunts also can require complex and expensive isolation amplifiers to convert the shut voltage to a value for input to a control circuit.
Conventionally, direct current and slowly changing currents can also be measured using Hall-effect detectors. Hall-effect detectors are expensive and have very low sensitivity in the 10 milivolt range. Having low sensitivity, Hall-effect detectors require shielded amplifiers for reliable detection in a noisy environment. In a noisy environment (such as that of a power converter), Hall-effect detectors have been found to be expensive.
Prior to this invention, the use of current transformers to measure direct current or slowly changing currents was not practical. Using current transformers to measure current on a line required an alternating field to couple the current in the line to the current transformer. Direct current or slowly changing currents alone did not have a sufficient alternating field to couple to a current transformer. Low frequency, alternating current, for example 50-60 Hertz, could be coupled to a current transformer if a low frequency current transformer was used. However, low frequency current transformers were large and expensive.
The present invention allows for the use of smaller and more economical high frequency current transformers instead of the large and expensive low frequency current transformers. It has been found in the present invention that chopping direct current or low frequency current into high frequency segments can be performed to couple the direct or low frequency current to a high frequency current transformer. It has also been found in the present invention that some circuits (such as those in power converters) inherently chop direct and low frequency currents such that chopping per se will not need to be implemented.
A problem with measuring chopped direct or low frequency currents is that the measured signal must be reconstructed or "unchopped" to its original direct or low frequency state. The present invention provides current detector circuits for reconstructing a signal to its original unchopped state faster, more economically and more precisely than heretofore possible.